A machine of this type normally comprises an electric motor and a hydraulic unit having a rotary impeller connected to the motor via a rotary shaft.
In order to prevent the medium within the hydraulic unit from entering along the shaft into the electric motor and damage it, one or several seals are arranged between the hydraulic unit and the electric motor. One common type seal used is a mechanical face seal which consists of one seal ring rotating with the driving shaft and one stationary seal ring attached to the surrounding housing. The two seal rings are pressed towards each other by spring force and prevent in this way liquid from passing between them.
It is however essential that a thin lubricating film is always established between the seal surfaces. This is necessary to prevent the seals from being damaged by dry friction. The consequence is however, that the necessary slot between the two seal surfaces is subject to a certain leakage. This leakage can normally be neglected and is harmless, but sometimes wear, vibrations or a wrong mounting may cause an increased leakage.
If the medium within the hydraulic unit contains contaminations, these may penetrate into the slot between the seal surfaces and cause damage which means that the sealing result is destroyed. It is therefore common to arrange two mechanical seals parted by a room filled by a barrier liquid such as oil. This means that the seal adjacent the electric motor, here called the inner seal, always operates with a clean medium, thus diminishing the risks for damages or the seal surfaces. If the seal adjacent the hydraulic unit, here called the outer seal, should be damaged, medium from the hydraulic unit could penetrate into the barrier liquid, but by checking the quality of said liquid at even intervals, the seal can be repaired before the liquid has been diluted too much. An example of this is shown in the Swedish patent No 366 013.